Sharing common ground between lead acid /lithium
Submitted: Monday, Nov 25, 2024 at 00:21
ThreadID: 149142
Views:1656
Replies:6
FollowUps:6
Justin72
Hi
Explorers,
I hope someone can give me some advice, I’ve seen a similar question posted but my situation is slightly different.
Wiring my Tinny and want to be able to keep existing crank battery to start motor and operate accessories, outboard will also charge this battery.
I want to add a battery selection switch to allow me to turn battery off, and to allow addition of my camping style lithium battery for longer trips.
Even when the lithium is switched on I will keep the motor connected to the cranking battery, only the accessories will be switched over to the lithium.
The motor is not rated to be started from a lithium battery, older model.
Lithium battery is big enough that I do not need it to be charged by the small motor alternator.
My question is: can I set my wiring so that both batteries share a negative connection - a shared ground.
This would mean that I connect positive for the lithium to one of the switch inputs, the negative from the Anderson plug connection to negative busbar which would also connect to lead acid battery along with all other accessories and motor.
I hope my question is clear and appreciate any assistance you can give.
J
Reply By: Member - Duncan2H - Monday, Nov 25, 2024 at 07:21
Monday, Nov 25, 2024 at 07:21
There is no problem at all with a common ground between the two batteries.. this is actually entirely normal.
Just make sure your switchover device is rated for more than the current that your accessories will draw.
AnswerID:
646859
Reply By: IvanTheTerrible - Monday, Nov 25, 2024 at 19:06
Monday, Nov 25, 2024 at 19:06
DONT use the hull as a ground
AnswerID:
646860
Reply By: RMD - Monday, Nov 25, 2024 at 19:26
Monday, Nov 25, 2024 at 19:26
J
It seems you are effectively wanting to use two different systems and circuits. If so. Why do you want to then have the negatives of those two circuits linked together?
If you still want to use the accessories from the start battery sometimes, it then would need a changeover switch which disconnects one systems Pos & Neg and then connects to the other source, ie, the desired battery source.
The changeover could be achieved by using a, (heavier duty, if needed) three position switch with centre OFF. One side could be to LITHIUM BATTERY and the other to Start battery. With accessories connected to the centre pins/connections of that switch, either battery source can then be selected at the flick of the switch. complete isolation of each battery and also from the HULL as mentioned.
AnswerID:
646861
Follow Up By: Tony W - Friday, Nov 29, 2024 at 06:45
Friday, Nov 29, 2024 at 06:45
Separate systems could result in stray earth currents, the last thing you would want in an aluminium boat.
All earths should be common with a solid negative rail bus / or connection.
FollowupID:
927612
Reply By: Member - FSH00 - Monday, Nov 25, 2024 at 21:16
Monday, Nov 25, 2024 at 21:16
Duel poll switch
AnswerID:
646862
Reply By: qldcamper - Tuesday, Nov 26, 2024 at 06:31
Tuesday, Nov 26, 2024 at 06:31
I'd just go with a common negative.
Why complicate things when you don't need to, a 2 pole high current change over switch isn't something you trip over every day and sounds expensive, also another 3 terminals that don't need to be there.
Common ground is quite happily used in motor vehicles with a lithium accessory battery so what's the difference?
As others have said, don't use the hull as a return. My time in the marine industry was spent on cement and glass hull vessels where every metal hull protrusion needed to be bonded to prevent electrolysis, that included the engine which is common to negative. I imagine an ally hull needs to be bonded to the engine too which in my time was always common to negative.
AnswerID:
646863
Follow Up By: RMD - Tuesday, Nov 26, 2024 at 07:38
Tuesday, Nov 26, 2024 at 07:38
QLD
The three position 2 pole switch is only for accessories so likely to be only a few amps draw. Jaycar sell them everyday and have plenty on their shelf. Just a few dollars and you have the option of either power source for those accessories. Perhaps I should have said not a MINI switch, just one which handles a few amps. It isn't carrying high starter current! Any inadvertent/mistake connection between the two systems then doesn't/cannot cause any issue, ie, burning the boat.
FollowupID:
927597
Follow Up By: qldcamper - Tuesday, Nov 26, 2024 at 08:23
Tuesday, Nov 26, 2024 at 08:23
Hopefully it won't be one of the un necessary negative terminals on the switch that causes the fire that takes out the entire Marina.
Try to stay within the relms of reality.
I simply gave the OP my opinion, if it differs to anyone else's opinion so be it.
Arguing with my opinion will not change it.
FollowupID:
927599
Follow Up By: RMD - Tuesday, Nov 26, 2024 at 16:21
Tuesday, Nov 26, 2024 at 16:21
QLD
How did you arrive at needing a HIGH CURRENT changeover switch? That was never mentioned. My alternative simply isolates one system from the other, and is easily within the realm of reality.! That is all!
There is no argument. Simply different ways to solve the problem. Both will work!
I cannot see how there is any argument at all.
PS I use some of those switches to change portable solar panel output from Parallel to Series when regs are not able to use the additional voltage.
FollowupID:
927603
Follow Up By: qldcamper - Tuesday, Nov 26, 2024 at 16:37
Tuesday, Nov 26, 2024 at 16:37
I don't care RMD, my opinion had nothing to do with your post.
The OP asked weather or not to have a common ground, it's obvious if he designed a system without a common ground he would need a 2 pole switch, the fact that you stated the plainly obvious in your post seems to make you think I'm directing my opinion at your post, get over yourself.
FollowupID:
927604
Reply By: drivesafe - Tuesday, Dec 17, 2024 at 15:17
Tuesday, Dec 17, 2024 at 15:17
Hi Justin and why not just leave the two batteries connected in parallel.
You probably do not have a SMART alternator, so both batteries will charge properly straight off the alternator.
Most of the starting current will come from the lead acid cranking battery and the accessories will run of the alternator while the motor is running and be powered from the lithium battery when the motor is off.
Plus, the lithium battery will keep the cranking battery fully charged when your boat is not in use.
It's a win-win setup.
AnswerID:
646972